Gate oxide integrity (GOI) testing is a critical metric for the effective functioning of a transistor. GOI testing is typically performed in the factory prior to shipment by applying a high stress bias voltage across the gate and source of an output power transistor of the amplifier, and measuring the leakage current associated with biasing the power transistor. The leakage current needs to be below a certain performance specification to be acceptable for shipment. However, GOI testing becomes more difficult when testing power transistors in an amplifier design, since other components (e.g., drive circuitry) associated with normal operation of the amplifier need to be protected during gate oxide testing. This may include providing additional isolation components that can effect the operation of the amplifier during normal operation.
For example, Class D audio amplifiers employ power transistors that are subject to gate oxide testing to meet acceptable DPPM (defects parts per million) specifications. Class D audio amplifiers are switching amplifiers that employ drive circuitry that switches between providing high supply voltage and a low supply voltage to the gate of a power transistor. If the power transistors are cascaded, a dedicated drive circuit may be employed for each power transistor. Additional power transistors and dedicated drive circuits may be employed for multiple channel devices. One technique for isolating a power transistor in an amplifier design for gate oxide integrity testing is to insert a diode between the final pull-up and pull-down of the gate driver. Another approach, is to couple the source terminal of the power transistor to a different pad for current measurements of the gate oxide. However, these techniques are not viable in a multi-channel class D audio amplifier due to the fact that the insertion of a diode in the pull-up path affects timing of the amplifier, and the limited bonding availability for independent paths due to area of the amplifier and pin count constraints.